I'm not grasping at anything. I asked a question early in the discussion and you answered it, but have now apparently changed your tune (or simply misunderstood my initial question). In either case it's fine; misunderstandings via internet forums are inevitable, and it's your prerogative to change your position. Once it became clear to me there was a disconnect I made my comment about apparently discussing different things....
The 10 second example would be an extreme case, however the same concept holds between two different players who manage to engage 0.5 sec after the pull and 4 sec after the pull. Assuming you have a competent tank and misdirects, there is nothing a shadow priest can open with that will pull aggro even if you begin casting immediately (barring a MB/SWD combo, which is clearly a ridiculous opener). For those of us who open with VT/MB there are no such worries as it is nearly 3 seconds after starting to cast before the first damage is caused. I see no reason to only consider a given players uptime when reviewing logs, and will continue to use the entire duration as the basis for comparison as I believe it does a better job of truly representing a players value in a given encounter.
It is apparent you reject this concept outright. This doesn't particularly surprise or interest me. I've given my justification for analyzing logs the way I choose to and have no wish to continue arguing over the internet. I do hope however that this discussion has perhaps provided some others who are less familiar with logs a means to start analyzing their own performance (using whichever method of considering uptime they find to be more appropriate).
If you wish you may now post for the last word as I won't be returning to this one unless there is an interesting change in the discussion